1. Field of the Invention
The present invention relates to a closed loop control system used in electric machines, and in particular electric generators. The control system is used to maintain an output voltage of the electric generator at a set-point value in a steady state condition. Additionally, the control system provides a timely response to changes in output voltage under transient conditions, such as a change in load or generator speed, to restore the output voltage to the set-point value.
2. Description of the Related Art
Various circuits have been introduced for use in closed loop control systems. As an example, U.S. Pat. No. 5,298,845 issued Mar. 29, 1994 to David P. DeBoer et al. describes a proportional plus integral controller operating an actuator in an HVAC system. This controller enables an integral part of a PI controller only when the error signal exceeds a predetermined value. A disadvantage of this method is that it does not use the output signal, in the case of the a DC generator the output voltage, to control the integral part of the PI controller. Instead, it controls the integral part based on a value of an error signal in relation to a threshold value. It is desirable to disable the integral portion of a PID controller during startup of the electric generator when the error signal has a maximum value and the output voltage is slowly rising to its set-point value. However, after startup it is desirable to include the integral portion of the controller even when the error signal has a maximum value, such as occurs when going from full load to no load, or vice versa.